Method of making a nib for a ball point writing instrument



Jan. 19, 1965 H. R. FEHLING ETAL 3,156,618

METHOD OF MAKING A NIB FOR A BALL POINT WRITING INSTRUMENT Filed Sept. 27, 1960 4 Sheets-Sheet 1 Fla. l

lwvewme Fig 2 HANS REINHfl-RD FEHL/UQ' Enwnp Hsum' HARVEY THE/R A-rroxnsyfls 3,166,618 FOR A BALL POINT TRUMENT Jan. 19, 1965 H. R. FEHLING ETAL METHOD OF MAKING A NIB WRITING INS Filed Sept. 27. 1960 4 Sheets-Sheet 2 FIG.3.

FIG 5 INVENTORS luv/Mp Fem-me E'owneo Hsuny llmzvsr B BY W Mam flTTom/5y5 HAus R:

Jan. 19, 1 H. R. FEHLING ETAL 3,166,618

METHOD OF MAKING A NIB FOR A BALL POINT WRITING INSTRUMENT Filed Sept. 27. 1960 4 Sheets-Sheet s j Im m-025 Hn-IIS RE/HI/AKDFEIlL/AIQ Eowmw Haney H/mvsr H. R. FEHLING ETAL METHOD OF MAKING A NIB FOR A BALL POINT Jan. 19, 1965 WRITING INSTRUMENT 4 Sheets-Sheet 4 Filed Sept. 27. 1960 FIG.8

INVENTORS HANS REM/HARD FEHL/NG AND EDWARD HENRYHARVEY BM, 3., mm

ATTORNEYS United States Patent-Qfifiee This invention relates to "writing extremities or nibs I swelling on absorption and shrinkage onloss of moisture.

for ball point writing instruments and, more particularly to new and improved methods for making nibs.

In most of the conventional ball tipped writing instruments the writing extremity or nib incorporates a socket or housing for a'writing-ball which latterpartly protrudes from the socket or housing but is rotatably held therein, without substantial play, between aninturned' lip anda base seat within the'housing and is supplied with a Writing substance (hereinafterreferred to as ink) through a feed duct leading through the base seat to the ball, the arrangement and disposition of the parts being such that as thefball is rotated such as by being moved over and in contactwith a writing surface such as a sheet of paper the ball carries a thin film of ink through the narrow gap formed between the ball and its housing which film is deposited onf said surfaceas a writing trace. "In general thebearing surfaces 'or 'sfeating surfaces provided in the housing for the ball are part-spherical and comprise a part-spherical lateral, seating surrounding the ball in the regionoftheball equator.and'a part-spherical base seat (which may be discontinuous, being dividedjby at least one ink channel leading outwards fromthe'lend of the feed duct). Typical designs of housing are shown in Unitedstates Patent No. 2,390,636. k

Proposals have, been made in the past to manufacture the nib :of such writing instrument from a. thermoplastic material. fThough this method is 'veryattractive in View of its obvious suitabilityfor mass production of an article atija low cost, all attempts .so far have failed to produce a'writing' instrument with a satisfactory writing per formance It was not only found that the trace was generally poor but also. that its quality changed duringlthe use of theilinstrument.

Wehave found'that'ithese "faults couldvnot even: be

eliminated by precision molding of nibs havinga suitable housing design ina'tough material like a synthetic polyamide resin, and that the principalreason for thepoor performance lies in the lack of rigidityof fall suitable. thermoplastic materials: Subsidiary reasons aredimen sional instability and'permeability to water Vapour of such materials, as well as the difficulty of producing a nib in suchmaterials which gives a. writing trace heavy enough to bejacceptable.

Lack of rigidityis a result of the low .modulus of elasticity of thermoplastic materials (1X10? to 5 10 p. s.i.) compared with that .of metals (1x10 to 5 10' p.s.i.). which are conventional materials for ball-point pen nibs In many cases .this low modulus is further reduced by absorptidnof moisture LDue .tothis lack of rigidity the socket or housing s I slightly distorted under the writing pressure sojthataffsufficiently even handwriting i l trace cannotbe produced under normal variations of writing angle and load. 7

Far from being of assistance, the resilient nature of such materials is their, main defect for the present purpose. A good writing performance depends on the main tenance of a very small but substantially uniform radial clearance. As has been established, this object is defeated by any significant elastic movement of the base seat and and/or thedistortiori bending of the lateral seat encircling the ball, when these seats and thereby the hous Permeability to water vapour is very detrimental where moisture setting inks are used. Most modern ball-point pen inks are now based on organic solvents Whichhave an afiinity'for water. If these modern moisture setting inks are contained in thermoplastic vessels exposed to normal temperate conditions, they absorb moisture through the walls of such vessels and suffer a decrease in viscosity which is most undesirable. Furthermore, the absorbed moisture may cause the precipitation of some constituents of the ink, and result in the blockage of the very narrow ink filled passages'adjacent to the ball thus I rendering the instrument useless. 7

Finally, it isdiflicult to produce a from plastic material which gives a writing trace of suflicient' depth. In order to form the seating sa'nd shape the rim of such a plastic nib to retain the ball,the nib usuallyhas to be heated by some means .and thus renderedinto a plastic "condition. In this plastic condition the housing is formed around the ball, or other spherical or part-spherical former, by the application of pressure. As the coeiiicient of thermal contractionof plastic materials is larger than that of metals, the housing, on cooling, shrinks tightly onto; the ball. Even when it'is possible torotate the ball when writing with such an instrument, the writing trace is very faint. i

In previous proposals it has been suggested that the slight grip exercised .On the ball -by'the resilient rim of a plastic 'nib is'an advantage, eg. in order to' avoid leakage *when th'e1pen is not in'use and in order to achieve some I kind of modulation in the depth of trace when varying load is applied to the pen during writing; We have found that such a grip is, on the contrary, most undesirable "and that, for asati sfactory' writing performance," it is necessary to effect accurate control ofthe. gap or clearance between the ball and itshousingin order to obtain the desired depth of trace together with complete freedom of rotation of the ball. The foregoing also applies when the nib isinjection-molded around a spherical or par't-spherical'former.

One'object o fthis'invention is to overcome these difiiculties, at least in part, and' to"; devise means ofproducing a ball-point pen nib from-a suitable thermoplastic material, such-that the result-ingwriting instrument performs satisfactorily. As used hereinafter the term nib refers either to the blank, or partially formed product of the molding operation,-or to the completely. formed writing 'tip, with or without the writing ball. With this object in View, the invention consists in a method of manufactur ing a nib 'for a ball point pen by injection molding from an injection-moldable material consisting of a suitablethermoplastic material which is soloaded with a finely *divided solid filler of suitable size and shape that the "elasticity modulus in compression of the molded material is not less than 6x10 -p.s.i'. and preferably exceeds '8 X10 p.s.i. By.injection moldable we mean that the loaded I material is capable of being injection molded. The thermoplastic material is preferably a' synthetic polyamide resinor similar wear resistant plastic having preferably an elastic-ity modulus in compression of not less than l l0 p.s.i. such as polycarbonate or acetal resins.

Patented Jan. 19, 1965 l pression of not less than 2x10 p.s.i. molded material contains preferably not less than 20 percent by volume of such granular filler. The filler material should be dimensionally stable and impermeable to water vapour. Apart from the filler the material may contain small quantities of additives, e.g., pigments and/ or low friction substances like graphite or molybdenumdisulphite. Copper powder, in head form, is a particularly satisfactory filler.

The invention provides the further step, in the manufacture of a ball point nib by the method above defined, of forming the part-spherical seating surfaces in the socket or housing subsequently to the molding operation by permanently deforming the material of the nib around a ball or ball-like member, inserted in the socket, under the application of pressure and of sutficient heat to render the material sufficiently plastic to conform to the surface of said member. Subsequent to the deformation of the material around the ball or ball-like member, the latter may be withdravm and a writing ball of slightly smaller diameter may be inserted. V

The invention also consists in providing a rigid'injection-moldable material suitable for articles manufactured by injection molding and having-an elasticity modu- .filler having an elasticity modulus in compression of not less than 2 1O p.s.i., and a melting point above the molding temperature. Preferably the thermoplastic material is a synthetic polyamide resin or similar wear resistant plastic, and the filler consists of substantially spherical particles. Polyhexamethylene-sebacamide is a particularly suitable material. 7

The invention further consists of a nib, for a ball point pen, at least the housing of which nib is manufactured by injection molding a mixture of a synthetic polyamide resin or similar wear resistant material as a matrix and a finely divided solid filler of suitable size and shape such that the elasticity modulus in compression of the molded material is not less than 6 l0 p.s.i. and preferably exceeds 8x10 p.s.i.

In this specification all figures for the elasticity modulus of plastic materials refer to the latter in the completely dry state at a temperature of 20C. This state can, for example, be attained by storing the material over calcium chloride for a sufiicient time. The material as molded is not necessarily'completely dry. 7 All metals commonly used in the manufacture of ball point pen nibs are sufficiently rigid to withstand any noticeable distortion under writing pressure. The elasticity modulus of brass and aluminum alloys is, for example, of theorder of 1X 10 p.s.i. On the other hand, all such metals are subject to significant wear due to the abrasive effect of dust and fibers picked up by the writing ball and entrained into the nib by its rotation.

Synthetic polyamide resins have considerable wear resistance and have in fact proved superior from this point of view to metals when used for the manufacture of ball point nibs. But the compression modulus of this class of materials is less than one tenth of that of metals.

Hence, a very considerable amount of stitfeningis required though, surprisingly, it has been found to be unnecessary to increase the modulus beyond about to l0 p.s.i., which is well below that of any metal suitable for nib manufacture. According to this inven tion this is achieved by filling the plastic material with a rigid filler. However, the thermoplastic material may easily lose its injection moldability if the concentration of solid filler is too high. Rigidity of the molded material can, therefore, only be increased at the expense of moldability, and vice versa.

We have found that the fractional volume occupied by the rigid filler in the filled material is not by any means the only factor deciding the ultimate rigidity of the filled material when molded. Particle shape and size distribution are very important, so that it is not generally possible to lay down hard and fast rules how to obtain good (and still less the best) results, in terms of a choice of values for the factors mentioned. We have, however, discovered that the control of both the lowest permissible fluidity to ensure injection moldability and the highest attainable rigidity obtainable with a given filler, the combination of which would obviously give the best possible result, can be obtained by the following method steps:

(l) Take a fine powder of a suitable rigid material and screen off all particles larger than approximately 0.002 in.

(2) Pour the dry powder into a suitable container and determine the maximum fractional volume occupied by the filler when packed in dry air, e.g., by shaking it down.

(3) Prepare a dense and uniform suspension of the filler in a suitable liquid, e.g. castor oil, such that the fractional volume of the filler is a little less than the maximum fractional volume in air as determined under (2).

(4) Test whether this suspension when uniform will flow in an even, though slow, stream without blockage when pressed out of a syringe with an orifice of, say, 34 inch internal diameter.

(5) If the test under .(4) is not satisfactory, diminish the fractional volume of the filler, still further e.g..by adding liquid, until the required minimum fluidity is achieved. I

(6) Prepare a molding powder in which the mixture of thermoplastic material and filler is so adjusted that the fractional volume of thelatter corresponds to the value determined under (5). 7

From this aspect the invention consists in a material suitable for injection molding consisting of a matrix of a suitable thermoplastic substance'filled with a finely dispersed rigid filler, the fractional volume of said filler in said material being more than 10% andless, but preferably not substantially less, than the maximum fractional volume of said filler in air. In order to be suitable, the viscosity of the unfilled thermoplastic liquid substance in the molten condition should be as low as possible, as it is otherwise impossible to fillit to the extent desired without obtaining a mixture whichcan-no longer be molded at a normal injection pressure. In this respect polyamides are particularly suitable as their viscosity is of the order of 1,000 to 2,000 poises at the temperature of molding. Other plastics like methylmethacrylate are not suitable for this reason as their viscosity in the unfilled state already approaches. the present limit of moldability which is of the order of 100,000 to 150,000 poises.

The invention further consists in a nib for a ball point pen manufactured by injection molding from such a material with a matrix consisting of a wear resistant thermoplastic substance like a synthetic polyamide resin and afiller consisting of a powder of oneor more ceramic and/or metallic materials with particles having a maximum linear dimension not exceeding 0.002 in. It-should, however, be, appreciated that this limit is nominal in the sense that no practical method of separation can ensure that a powder contains no particles above a certain size; nor is this essential for present purposes, as it is only necessary to ensure that the thinnest sections of a nib are uniformly filled with a homogeneous mixture of the material.

The Wear resistance of a filled thermoplastic substance cannot be predicted Without experiment. Polyamides have given the best results so far. When filled according to the examples given below, the wear resistance has been substantially higher than that of present brass or bronze nibs. On the other hand, cellulose acetate and.

acetate-butyrateplastics have been found unsatisfactory which ofv all the. suitablethermoplastic substances at' present available, has the highest fluidity in molten form. In this case, perfect moldability was retained up to 55% by volume of copper or glass beads, the limit being reached at 6.1% (see Example 4 below).

of the type used. in metallic paints or of mica powder was limited to approximately byvolume due to progressive .deterioration" of the moldability for higher concentrations; However, the1relative, increase in elasticity modulus per one percent additionof filler is also substantially higher than for aisphcrical filler.

For these reasons no hard and fast rule can be given about the suitability'of a particularshape of filler with- ,out a test on the lines indicated above. But for roughly 3 On the other hand, the addition of fine copper flakes spherical-particles thefollowirig table .will afford general guidance:

Filler content,'percent by volume..." 0 Relative increase in vis''osity Relative increase in compression modu- High adhesion; 1 Low adhesion 1 e 4.1 4.8 as- 3 2.5 2.8 3.1

Fillers. 'Copperpowder, in bead form,'manufactured atomisationlfrom pure electrolytic copper (manufactur erg' Powder Metallurgy Ltd); passed through U.S.

'Sta'ndard'sieve 400 mesh; approx. range of particle size 0.0001 to;- 0.0015";maximum fractional volumein filler- 45%; I i

fcomp're'ssionmodulus, dry: 11 X 10 psi.

-" =Matr ix.-' B 100 Nylon as in Example l. p

Fi ller. Glass powder, in head form, manufactured from soft soda glass :(manufacturer: Ballotini Manufac turing 00., Ltd., Barnsley, Yorks.); passed through US.

Standard sieve 40'0mesh; approx. range of particle size 000021to- 0.0=015"; fractional volume in air=58 %.j w

lrijetion molded material. Fractional volume of Compression modulus, dry: 8.5 10 p.s.i.

EXAMPLE 3 Mam -:13 100N916 as in-Examp-le 1.

Filler.-Mica powder, in flake form, manufactured ifrom pureMuscovite mica (manufacturer: The Central PulverizingCo, Ltd. London); designated 150/3 mica powder, 300 mesh; approx. range of'par-ticle size:'0.0001 to 0.002"; maximum fractionalvolume in air =l2%. .,Injeqtion-m0lded material; Fractionalygvolume of .fil1er, ='11%. V, I

material. Fractional volume of Compression modulus, 8.510 p. s.i.

"pens.

Compression modulus, dry: 9 X 10 p.s.i. EXAMPLEA Matrix.%B 10o Nylonas-in Example 1.

Finch-Copper beads (as in Example 1); particle size range: between .0015" (400 mesh) and .002 270 mesh); maximum fractional volume in air=64%.

, Injection-molded material. Fraction-a1 volume of.

filler 61%.

Compression modulus, dry: 25 to 30x10 p.s.i.

EXAMPLE '5 EXAMPLE 6 Matrix.Polycar bonic acid ester of 4,4-dihydroxy-diphenyl-2,2 propane (Markrolon Grade S of Farbenfabriken Bayer, Leverkusen compression modulus: 3.2.x 10 p.s.i.; maximum chip size: 16 mesh.

Filler.C0pper heads (as in Example 4).

v, lnjectionmolded ,;materials. Fractional volume of fi1 ler:l 39%. v i

We have also observed that copper spheres yield-better results than glass sphere s as fillers for a synthetic polyamide material. V It app'ears thatcopper adheres. to the latten after injection moldinglwhile glass and bronze powders arefarfinferior in this"respect- Thegadhesion of j silver approaches that ofipure copper. Whether this is due to the chemical differences between these filler materials or to any differences in their surface structure could not be established. Though in both cases the plastic "materiahshrinks on to the solid filler after solidifi: cation'and cooling,"it is plausible that any firm adhesion between the filler and'the matrix will enhance the rigidity of the molded material, and yice-versa.-- It' is obvious that adhesion iseof less importance where the material is used in 'compression'only which is' very' nearly the case in a plain bearing. The permeability tojwater vapour is stronglyreduced and the dimensional tability increased by using-.heavily'filled materials accordingto the inven- The permeability of'th'e. filled Nylon, according to.

tion. the above examples was reduced to about one'third of the value for the unfilled Nylon which was found to be sufiicient for the use of this materialin the'field of ball point No distortion of the. nibs made of thi material due to moisture absorption could be detected. FIGURE 1' shows in section, and on a large scale, a writing extremity or nib, of the ball point type, suitable for manufacture by injecting molding, While FIGURE 2 is a diagram, on a still larger scale, illustrating the dispersal of spherical particles within the.

plastic matrix;

FIGURE 3 illustrates the molding process and parts of the tools; w i I wFIGURE 4- is a perspective view, partly brokenaway, illustrating the configuration of the socket as molded;

FIGURE 5 is a similar perspective view.illustrating the configuration of the socket after the formation of the part-spherical seating surfaces;

FIGURE 6 is a greatly enlarged cross-sectional view through th'e'base seat of a nib prepared according to the invention and fitted with a cratered ball;

FIGURE 7 is a schematic sectional view illustrating one state.

scription. The writing ball 2 (usually 1 mm. diameter) is rotatably retained in a'socket or housing 3, without substantial play, by an inturned lip 4 on the nib, the ball being positioned between this inturned lip and a base seat consisting of a plurality of circumferentially-spaced, part-spherical, seating surfaces 5; it is held laterally by a part-spherical lateral seat 6 which. encircles it above and below the equatorial plane. The lateral seat 6 is spaced from the base seat by an annular cavity 7 which surrounds the ball; there is an ink feed duct 8, 8a, which leads to the submerged pole of the ball and ink channels 9 lead from this duct, between the base seating surfaces 5, to the cavity 7. The ball may be cratered ball a large-diameter upper part 10a which produces the substantially-cylindrical main portion of the socket, joined by a frusto-conical portion 10b to a stub 100 which produces part 8a of the feed duct 8. The frusto-conical portion 1012 has a plurality (e.g. five) of circumferentially-spaced concave depressions 13 which may be formed by spark erosion. In the'molding operation, these depressions 13 result in complementary mounds or pimples 14 on the interior of the socket, as illustrated in FIG. 4, with the channels 9 extending between them from the feed duct 8,- 8a, to the substantially-cylindrical part of the housing. Nibs according to this design could be man'- ufactured by injection molding the materials specified'in the above examples, under the following working conditions:

Injection pressure 510,000 p.s.i.

Length of sprue 0.5" (12 mm.). Diameter of sprue 0.065" (1.6 mm.). Size of square gate 0.055 (1.4 mm.). Injection temperature for materials based of B 100 Nylon 285 C. Mold temperature for materials based on B 100 Nylon 150 C.

Comparatively large gates, as well as high injection and mold temperatures according to the above values have been found very desirable in order to achieve moldings of high quality.

The right hand partof FIG. 1 indicates how the particle size of the filler is limited by the configuration of the housing. For the design illustrated in FIG. 1 the preferred filler consists of spheres or substantially spherical beads 15 with a maximum diameter of 0.0015" (or approximately 0.040 mm.=40 microns). FIG. 2 illustrates the idealized arrangement of such spheres within the plastic matrix for a fractional volume of 50% and for rhombohedral packing, in which case d=0;14D.

The actual arrangement of the spherical particles in the solidified material can best be described as a uniform random distribution not differing in its essential characteristics from the idealized arrangement shown in FIG. 2. It will be appreciated how close this packing is, and why a very large increase in rigidity can be obtained without destroying the mobility of the mixture in the molten It will be equally clear that the dimensional stability and resistance to permeation by water vapour are greatly increased, if the filler consists of a ceramic material (e.g. glass) or a metal (e.g. copper).

In the present manufacture of ball point nibs the ball has to be retained in its housing by permanently deforming the housing walls round the ball such that they assume a spherical interior shape conforming to the hall.

For metal nibs this is done by impact with a conical die (peining), by spinning or any similar method suitable for the accurate cold deformation of ductile metals.

In view of the large elastic strain which plastics can undergo such methods are usually unsuitable for nibs in which the plastic material forms the matrix because the deformation of the housing wall in the retaining operation does not stress the material beyond the elastic limit and is therefore not permanent. It is, therefore, necessary to lower the elastic limit of the material which is most conveniently done by heating it.

The best method of hot deformation of the housing walls to produce both the-part-spherical base seat 5 and lateral seat 6 for the ball, illustrated in FIGS. 7-and 8, is the following:

An oversize ball of, say, 1.005 to 1.010 mm. is inserted into the open socket of the nib molding which is firmly positioned on a suitable anvil 19. This ball therefore rests on the mounds 14. A heated die 20 with a conical recess 21 is brought into contact with the ball and the lip portion of the nib molding while in longitudinal alignment with the axis of the die and of the nib housing. Axial loading is applied to the die over a short period which suffices to convert the sections of the nib Material B 100 Nylon.

Loading .45 %'-by volume of copper spheres. Included angle of die degrees. I Die'temperature 200 C.

Load 6 lbs.

Duration of load 5 seconds.

After the nib has cooled to room temperature the oversize ball is pushed out of the housing, past the inturned lip 4, by a suitable needle passing through the feed channel at the rear of the ball. A smaller ball of, say, 1,000 mm. is replaced by pushing it past the lip into thehousing.

This procedure is necessary as the plastic material shrinks tightly on the metal ball in cooling subsequent to the process of hot deformation. Hence, the replacement by a ball 10 microns smaller in dia. does not lead to the formation of a radial clearance of 5 microns but to a considerably smaller clearance depending on the stresses set up during the hot deformation and the physical properties' of the material used. The correct oversize may easily be determined by experiment. It will be appreciated that it is very easy to obtain complete and accurate control over the depth of trace wanted by the simple device of varying the size of the ball replaced into the nib, or for preference varying the 'size of the ball used inhot deformation.

Certain plastics like the polycarbonate resin in Example 6, are capable of being permanently deformed without the application of heat. As in the case of metal nibs, the operation of peining does not jam the ball in its housing: due to the release of the elastic stresses set 'up during peining a radial clearance is formed between V with materials which, like synthetic polyamide resins, are

not brittle and can undergo a high enough elastic strain in the cold state. Owing to the re-entrant configuration of the housing lip after the hot deformation, the lip has to undergo a transient elastic deformation to allow removal and substitution of the ball. The tensile strain which the material undergoes in this operation is approximately as follows:

Ball protrusion, percent of ball dia 36 32 Total tensile strain approx. percent. 4

We have found that B 100 Nylon material loaded with copper or glass spheres up to 55 percent by volume is capable of withstanding this strain without fracture or significant permanent deformation. On the other hand, comparatively brittle thermoplastics like styrenes are unoccupying about 40% of the ball surface and being pro- V vention as defined by the following claims.

suitable in this respect, so that the ball replacement suited for the manufacture of nibs according to the present I invention because it combines low melt viscosity, high wear resistance, dimensional stability and high elastic strain. We found that B 100 Nylon, due to its high dimensional stability was superior to the A 100 grade (polyhexamethylene adipamide) though the latter has a higher elasticity modulus. The only drawback of these polyamides, i.e., their relatively poor adhesion to many materials, is overcome by the choice of a metal like copper or silver, both of which are also excellent bearing metals in a nib for a ball point pen.

The best writing performance of nibs manufactured from this combination of materials was obtained by replacing the oversize ball used for deforming the lip by hot peining by a cratered ball as described in the copending United States application of Hans R. Fehling and Edward H. Harvey, Serial Number 29,608, filed May 17, 1960, and as illustrated in FIGURE 6 of this application. Preferably the difference in diameter between the oversize ball and the cratered ball should be 8-10 microns. The cratered ball should preferably have craters of 40 to 50 microns in diameter covering about 40% of the ball surface. Best results have been obtained by using, in such a nib, an ink having a viscosity between 50 and 100 poises at the normal temperature of writing.

The base-seat of a nib 1, equipped with a cratered ball 2', is illustrated on a much-enlarged scale in FIGURE 6. It will be seen that the smooth surface 17 of the ball is pitted with a multiplicity of craters 16 of substantially uniform area dispersed over the whole of said surface. Therefore, in addition to the radial clearance or gap between the smooth spherical surface of the ball and the part-spherical seating surfaces (such as 5) through which gap the ink is carried in shear by' rotation of the ball, the craters 16 carry ink bodily from the housing to the writing surface irrespective of the size of the radial clearance. The craters are produced by spark-erosion in accordance with the said co-pending patent application. In a suitable example the ball is a polished stainless steel ball 1 mm. in diameter pitted with craters of about 40 microns in diameter and about 6 microns deep, the craters We claim: V

.1. A method for making a nib for a ball point writing instrument comprising the steps of preparing a deformable plastic material loaded with at least ten percent by volume of a finely divided solid filler material so that the resultant mixture is capable of injection molding and has an elasticity modulus in compression of at least 6X10 p.s.i. when dry, and injecting the mixture into a mold to form a nib housing having a central ink duct, a ballreceiving socket communicating with the ink duct provided with a plurality of base seat mounds surrounding the ink duct and a rim portion adapted to be deformed to retain a ball in the socket.

2. A method according to claim 1 including the steps of removing at least the socket-forming part of the mold and inserting a die member having a spherical surface into the socket and applying sufficient pressure to permanently deform the base seat mounds to provide partspherical base seating surfaces thereon for theball.

3. A method according to claim 2 wherein the plastic material is a thermoplastic and including the step of applying heat to the socket through the die member simultaneously with the application of pressure.

4. A method according to claim 2 wherein the rim portion forming part of the mold is also removed and including the step of applying pressure to the exterior of the nib housing to deform the rim portion so as to'form an internal part-spherical lateral seating surface for the ball.

5 .-A method according to claim 2 including the steps of withdrawing the die member and inserting into the socket a writing ball having a slightly smaller diameter than thespherical surface of the die member.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD FOR MAKING A NIB FOR A BALL POINT WRITING INSTRUMENT COMPRISING THE STEPS OF PREPARING A DEFORMABLE PLASTIC MATERIAL LOADED WITH AT LEAST TEN PERCENT BY VOLUME OF A FINELY DIVIDED SOLID FILLER MATERIAL SO THAT THE RESULTANT MIXTURE IS CAPABLE OF INJECTION MOLDING AND HAS AN ELASTICITY MODULUS IN COMPRESSION OF AT LEAST 6X10**5 P.S.I. WHEN DRY, AND INJECTING THE MIXTURE INTO A MOLD TO 